1. Field of the Invention
The invention relates generally to a method of testing an internal combustion engine and particularly to a method and apparatus for determining the relative compressions of the cylinders of an engine; the starter motor current or an electrical magnitude proportional to it being evaluated while the ignition is suppressed.
2. Prior Art
A method and apparatus for evaluating the relative compressions of cylinders of an internal combustion engine is disclosed in German Patent No. AS 2 709128. In this disclosure the current in the starter motor is employed as a measure of the variation in pressure in the individual cylinders with the ignition suppressed during the period of measurement. The measured current or an electrical magnitude proportional to it has superimposed on it a varying component which arises from the pulsating opposing torques on the crankshaft as each cylinder undergoes a compression stroke and an expansion stroke during the starting process performed with the ignition suppressed. Accordingly, it allows a comparison of the compressions of the individual cylinders.
This known method has a serious drawback which lies in the fact that, in the measured starter motor current or the electrical magnitude proportional to it, the influences of the individual cylinders of the engine are not in proportion. For example, when a cylinder has a poor compression, it can arise that in the cylinder which follows it in the engine cycle, an excess of current is used to cause compression in that following cylinder which results in a distortion of the relative difference in compression measurements between the two cylinders. Accordingly, the difficulty in locating a fault and overcoming it increases.
In the above-mentioned German patent, to correct the effects of this disproportionate representation in the evaluation of the measurements, an adjustment is made whereby the integral value of the alternating portion of the starter motor current is calculated by integrating along a time axis connecting the minima. Then, to correct matters, the difference between the integral value of a complete cycle and the integral value of the current draw curve up to the preceding cylinder is added to these integral values with its sign reversed. In this way, account is taken of the theory that the above-mentioned error arises through a loss in the amount of work stored in the compressed air; i.e., when a cylinder has a deficient compression, the next following cylinder in the engine cycle needs an increase in the starter motor current because part of the energy stored in the air which should have been compressed in the preceding cylinder is lacking as a consequence of the poor compression. However, it has been shown that even using the stated adjustment of the known method it has not been possible to achieve satisfactory accuracy or reliability in measurements on different engines and in measuring normal operation.